The steps of making printed circuits from metal clad insulating bases by the so-called print and etch technique are well-known and are described, for example, in U.S. Pat. No. 3,269,861. Such methods are laborious and expensive in that they call for providing a relatively costly metal foil on a substrate, printing the foil with an acid-resistant mask to define a pattern, etching the exposed metal with an acid to remove it from the surface and finally stripping the resist to expose a conductive pattern of metal foil. In addition to being expensive and time-consuming to produce, printed circuits made by such processes have conductor patterns which are not strongly bonded to the base -- the limiting factor being the efficiency of the adhesive used to bond the foil to the substrate -- and this has been subjected to etching and stripping solutions.
In such print and etch techniques, the circuit diagram can be formed on the base panel in a variety of ways. In a photographic modification of the print-and-etch technique, a light sensitive enamel, such as a proprietary photoresist, is uniformly spread over the clean metal foil and dried. Then a pattern negative is placed over the coated panel and, and after exposure to a suitable source of light, the exposed areas of enamel are hardened. Immersion of the panel in a solvent for the non-hardened areas dissolves the unexposed enamel away from the base leaving the metal foil showing. The portions of foil not protected by the hardened enamel are etched. Following etching, the exposed, hardened light sensitive enamel is stripped from the panel with an appropriate solvent to produce the printed circuit board.
In addition to the other disadvantages in the print and etch techniques, as noted above, such a photographic system is slow and expensive in long production runs and therefore resist printing is conventionally carried out instead either by offset printing or by screen stencil printing, both of which provide limitations on line thinness, and resolution.
A method has now been discovered to radiant energy from the patterns which eliminates the need for resists and etching, thereby eliminating the onerous stripping steps heretofore employed. In addition, the pattern produced is capable of being intensified into a conductive pathway by conventional metallization techniques, and thus there is no need to attach a foil to the base and etch away the uneeded areas to define the patterns. The most recently evolved techniques of unclad resin bonding and for direct (adhesiveless) bonding are applicable with the new technique, and bond strengths between the conductor lines and all conventional non-polar substrates are superior to what are obtained from conventional adhesive backed foil clad substrates. Moreover, by providing holes in the substrate, in strategic locations, and irradiating the hole walls as part of producing the printed circuits, through-hole electrical connections may be made non-mechanically, between circuit patterns on opposite sides of the insulating base. Finally, because the negatives can have very thin lines, the conductor lines can be very thin too, and the resulting circuit patterns can be highly resolved with line thinness limited only by overhang of the metal deposit.